Due to the continued reduction of petroleum resources, the need to look for alternative resources or feedstocks, process routes and materials has become a scientific aim for the chemical industry. The use of renewable biomass as sustainable resource for the production of bio-fuels and bio-chemicals is expected to grow significantly in the coming decades. This expansion is enabled by the breakthroughs in genomics and industrial biotechnology. The so-called white biotechnology area focuses on alternative production routes for producing organic compounds such as carboxylic acids and alcohols from renewable bio-based feedstocks such as glucose, starch, non-edible oils and fats, woody and agro-food residue streams or organic waste streams containing these feedstocks. The industrial biotechnology makes use of enzymes or micro-organisms to convert the mentioned feedstocks, possibly after a pre-treatment step, into the targeted bio-fuel or bio-chemical. White biotechnology may reduce our dependency on oil and has the additional advantage that such routes are considered as CO2-neutral as CO2 has been captured in the biomass during the growth.
Fermentation is the most dominant technique within the white biotechnology domain. Fermentation is a metabolic process in which organisms like yeast, bacteria or fungi convert feedstocks like sugars, fatty acids or glycerol to products like acids, gases and/or alcohol.
Besides the fermentation product itself, fermentation streams contain large amounts of water and many secondary compounds. This makes the recovery of the desired product often difficult.
A known process that is often used for recovering organic products from a fermentation stream is liquid-liquid extraction. In this process, the organic product (the solute) is extracted from the aqueous fermentation stream into an organic solvent (the extractant), thereby obtaining a mixture mainly comprising the organic fermentation product and the organic solvent (this mixture may be referred to as the extract or enriched solvent). The organic solvent is subsequently regenerated by a back extraction step in which the organic product is transferred to water, thereby obtaining an aqueous product solution and the regenerated organic solvent. The pH of the water stream entering the back-extraction is often adjusted to stimulate the transfer of the organic compound from the solvent to the water stream. The aqueous product solution is then typically purified, e.g. by crystallization. Various methods are known to achieve crystallization, such as adjustment of the pH, cooling or the removal of water by evaporation and/or distillation and or combinations of the aforementioned methods.
A disadvantage of back-extraction is that this step requires high energy or the addition of extra chemicals to successfully transfer the organic solvent back into water. In order for the organic compound to leave the organic solvent and enter the water, the solubility of the organic product in water needs to be enhanced. This may for example be achieved by changing the temperature or the pH of the enriched solvent. However, this will require high energy costs due to heating and/or cooling (in case of changing the temperature) or adding chemicals (in case of adjusting the pH).
A further disadvantage of back-extraction is that the aqueous product solution may get contaminated by the organic solvent. Similarly, a disadvantage of extraction is that the feed stream may get contaminated with organic solvent.
A further disadvantage of back-extraction is that the resulting aqueous solution comprising the organic product will typically be rather diluted and comprises a relatively large amount of water. This water needs to be removed in the down-stream processing.
A further disadvantage is that the organic product may not yet be in the desired form after back-extraction. An illustrative example of the last feature is the back-extraction of organic acids into water with a relatively high pH (i.e. >>pH=7), which results in the conversion of the acid into the corresponding carboxylate during the back extraction. Recovery of the acid from the aqueous product solution will then often require a pH adjustment with an acid to convert the carboxylate back into the targeted acid. Such a procedure does not only lead to high costs for acid and base consumption but also to the production of a salt as a by-product, where the salt is composed of the counter-ions of the used acid and base.
It follows from the above that there is a need to develop short, simple and cheap recovery processes to recover organic products from feed streams, in particular from fermentation streams.